JavaScript Boolean

falsy

Considered as false(falsy)

• false
• null
• undefined
• NaN
• 0
• ’’

Considered as true(truthy)

• []
• {}
• everything else except falsy

logical operators

Logical operator in JavaScript, you will get not only true, false, and also a value that you compared. For example,

console.log("JS" && "JavaScript");

Both are true, so it seems like it returns true, but you will get “JavaScript”.

In JavaScript, logical operators work like

left side of value	and	or
truthy	right-hand side	left-hand side
falsy	left-hand side	right-hand side

console.log(true && true); // true
console.log(true && false); // false
console.log(false && true); // false
console.log(false && false); // false

console.log(true || true); // true
console.log(true || false); // true
console.log(false || true); // true
console.log(false || false); // false

console.log(null && undefined); // null
console.log("0" && NaN); // NaN
console.log({} || 123); // {}

You can use this method like

function print(value) {
  const message = value || "Hello, world!";
  console.log(message);

  print(); // Hello, world!
  print("Javascript"); // JavaScript
}

In the code, value ‘

’ “Hello, world!” is that if value is not given, it will return “Hello, world!”(right-hand side) since ‘

’ operator return right-hand side if left-hand side is falsy.

nullish coalescing operator

The nullish coalescing (??) operator is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

const title1 = null ?? "Hello";
const title2 = false ?? "Hello";

console.log(title1); // Hello
console.log(title2); // false